Fig. 6.4 Reflex pathways involving skeletal muscle (Source of image below. Used with permission.)


When sensory neurons in the skin of the left foot detect the intense pressure caused by stepping on a sharp object, their dendrites carry out (1) reception. This causes the dendrites to (2) conduct impulses up through the nerve in the leg. These impulses reach and enter the gray matter in the back of the spinal cord via the sensory neuron axons, which (3) transmit them through synapses to other neurons in the spinal cord gray matter. Since these next neurons extend from one neuron to another, they are called interneurons. The interneurons (4) transmit the impulses to somatic motor neurons in the front part of the gray matter of the spinal cord. The impulses are then (5) conducted down the motor axons in the nerves in the left leg to certain muscles in the thigh and calf. Neurotransmitters from the motor axons (6) stimulate these muscles to contract, causing the response of lifting the foot and thus relieving the intense pressure and protecting the foot from harm.


Proper reflex responses may require coordination in addition to monitoring, communicating, stimulating, and unconscious remembering. For example, to prevent loss of balance when lifting the foot, cooperation by a second reflex must occur. Branches of the sensory axons transmit impulses to other interneurons that cross over to the right side of the spinal cord. These crossing interneurons (7) transmit the impulses to other somatic motor neurons in the right side of the gray matter. Impulses in these motor neurons are (8) conducted down the nerves in the right leg. The impulses cause certain muscles in the right leg to contract, resulting in a straightening of the right leg at the same time that the left leg is bending and lifting the foot off the object. In this way, the right leg supports the weight of the body so that the person does not fall down.


Another aspect of coordination is shown by the withdrawal reflex. As the interneurons stimulate motor neurons to the muscles that will make the appropriate actions occur, the interneurons send (9) inhibitory impulses to motor neurons controlling leg muscles that would interfere with the proper movements. This prevents antagonism among the muscles.





  Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland
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Source of image. Used with permission.


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